Media synchronisation system

ABSTRACT

A communications system distributes code word pairs within the audio of a television or radio program or the like. Each pair of code words includes an ID code word that is the same for a given program and a synchronization code word that is unique within the program. A portable user device is able to synchronize itself to the program using the embedded synchronization code words.

This application is a National Stage Application of PCT/GB2013/050956,filed 12 Apr. 2013, which claims benefit of Serial No. 1206564.5, filed13 Apr. 2012 in Great Britain and which applications are incorporatedherein by reference. To the extent appropriate, a claim of priority ismade to each of the above disclosed applications.

BACKGROUND

This invention relates to methods and apparatus for embedding code wordsin an audio signal and to methods and apparatus for synchronising adevice to the audio signal using the embedded code words. The inventionhas particular, but not exclusive relevance to communications systems inwhich a portable user device, such as a computer tablet or a cellulartelephone, is provided with data via an acoustic data channel.

The applicant has proposed a number of acoustic data communicationsystems. Examples of earlier systems can be found in WO2008/145994,WO2009/144470 and WO02/45273, which describe systems in which hiddendata can be transmitted to a portable user device (such as a cellulartelephone) within the audio of a television or radio programme that maybe being delivered over a broadcast channel or from a local storagedevice (DVD etc). In the present context, the data is hidden in thesense that it is encoded in order to try to hide the data in the audioso that it is not obtrusive to the user. As those skilled in the artwill appreciate, the acceptable level of audibility of the data willvary depending on the application and the user involved. Varioustechniques are described in these earlier patent applications forencoding the data within the audio, including spread spectrum encoding,echo modulation, critical band encoding etc. Any of these data hidingtechniques may be used in the present invention.

SUMMARY

One aim of the invention is to provide alternative, and in some waysimproved, techniques for hiding the data in the audio and techniques forrecovering the hidden data from the acoustic sound.

According to one aspect, the present invention provides a method ofembedding code words into an audio signal, the method comprising:receiving pairs of code words, each pair of code words comprising an IDcode word and a synchronisation code word; encoding the code words ofeach pair of code words separately to form encoded code word signals;receiving an audio signal; and combining the encoded code word signalswith different portions of the received audio signal to form a modifiedaudio signal that carries the pairs of code words distributed within theaudio signal.

In one embodiment, the ID code words of all the pairs of code words fora given audio signal are the same and each synchronisation code word forthe audio signal is unique. The pairs of code words may be continuouslyencoded within the audio signal or they may be distributed throughoutthe audio in a distributed manner.

Where the audio signal comprises one or more break points, one or moreadditional synchronisation code words are preferably encoded into theaudio signal after a break point, to facilitate re-synchronisation afterthe break.

The encoding step typically generates timing information associated withat least the synchronisation code words that identifies the point intime where the code word is encoded within the audio signal. This timinginformation may then be provided to a receiving device for use insynchronising the device to the incoming audio.

The invention also provides a method of synchronising a device with anaudio signal having encoded pairs of code words embedded therein, eachpair of code words including an ID code word and a synchronisation codeword, the method comprising: receiving the audio signal with the encodedpairs of code words; and synchronising the device with the audio signalby detecting: i) an ID code word and a synchronisation code word fromone pair of code words; or ii) an ID code word from one pair of codewords and a synchronisation code word from another pair of code words.

The method may further comprise using a detected ID code word toretrieve timing information relating to an expected time for detectedsynchronisation code words and determining a difference in time betweenan expected time for a detected synchronisation code word and an actualtime of the detected synchronisation code word and using the determineddifference to synchronise the device to the audio signal. In this case,the timing information may define an expected time relative to a definedpoint within the audio signal, such as the start of the audio signal.The determined difference may be used to alter a timer of the device.

Action information may be stored in the device that defines one or moreactions that the device should take at defined times during thereception of the audio signal.

In one embodiment, a first decoding technique is used to decode theencoded code words in the received audio signal before synchronisationand a second decoding technique is used to decode the encoded code wordsafter synchronisation.

In another embodiment, a first decoding technique is used to decode theencoded code words in the received audio signal and a second decodingtechnique is used to decode the encoded code words in the event that anexpected code word is not detected.

The first decoding technique may be a blind decoding technique and thesecond decoding technique may be a non-blind decoding technique.

In one embodiment, after synchronising the device to the audio and inresponse to a skip within the received audio signal, maintaining the IDcode word and re-synchronising with the audio signal by detecting asynchronisation code word. The skip may be a fast forward or a rewindoperation.

The invention also provides an apparatus for embedding code words withinan audio signal, the apparatus comprising: means for receiving pairs ofcode words, each pair of code words comprising an ID code word and asynchronisation code word; means for encoding the code words of eachpair of code words separately to form encoded code word signals; meansfor receiving an audio signal; and means for combining the encoded codeword signals with different portions of the received audio signal toform a modified audio signal that carries the pairs of code wordsdistributed within the audio signal.

The ID code words of all the pairs of code words for a given audiosignal may be the same and each synchronisation code word for the audiosignal may be unique.

The pairs of code words may be continuously encoded within the audiosignal.

Where the audio signal comprises one or more break points, the means forcombining the encoded code word signals combines the signals so that oneor more additional synchronisation code words are encoded into the audiosignal after a break point.

In one embodiment, the means for encoding generates timing informationassociated with at least the synchronisation code words, which timinginformation identifies the timing within the audio signal where the codeword is encoded.

The present invention also provides an apparatus for synchronising adevice with an audio signal having encoded pairs of code words embeddedtherein, each pair of code words including an ID code word and asynchronisation code word, the apparatus comprising: means for receivingthe audio signal with the encoded pairs of code words; and means forsynchronising the device with the audio signal by detecting: i) an IDcode word and a synchronisation code word from one pair of code words;or ii) an ID code word from one pair of code words and a synchronisationcode word from another pair of code words.

The apparatus may further comprise means for using a detected ID codeword to retrieve timing information relating to an expected time fordetected synchronisation code words; means determining a difference intime between an expected time for a detected synchronisation code wordand an actual time of the detected synchronisation code word and meansfor using the determined difference to synchronise the user device tothe audio signal.

The timing information may define an expected time relative to a definedpoint within the audio signal, such as the start of the audio signal.

The apparatus may also comprise means for using the determineddifference to alter a timer of the device. Action information may alsobe stored that defines one or more actions that the device should takeat defined times during the reception of the audio signal.

In one embodiment, the apparatus has means for using a first decodingtechnique to decode the encoded code words in the received audio signalbefore synchronisation and means for using a second decoding techniqueto decode the encoded code words after synchronisation.

In another embodiment, the apparatus has means for using a firstdecoding technique to decode the encoded code words in the receivedaudio signal and means for using a second decoding technique to decodethe encoded code words in the event that an expected code word is notdetected.

The first decoding technique may be a blind decoding technique and thesecond decoding technique may be a non-blind decoding technique.

The apparatus may further comprise means for re-synchronising the devicewith the audio signal in response to a skip event within the receivedaudio signal by maintaining a previously decoded ID code word and bydetecting a new synchronisation code word after the skip event. The skipevent may be a fast forward event or a rewind event.

The present invention also provides an apparatus for embedding codewords within an audio signal, the apparatus comprising: an encoderconfigured to receive pairs of code words, each pair of code wordscomprising an ID code word and a synchronisation code word andconfigured to encode the code words of each pair of code wordsseparately to form encoded code word signals; and a combiner configuredto combine the encoded code word signals with different portions of thereceived audio signal to form a modified audio signal that carries thepairs of code words distributed within the audio signal.

The present invention also provides an apparatus for synchronising adevice with an audio signal having encoded pairs of code words embeddedtherein, each pair of code words including an ID code word and asynchronisation code word, the method comprising: a receiver configuredto receive the audio signal with the encoded pairs of code words; and asynchronisation controller configured to synchronise the device with thereceived audio signal by detecting: i) an ID code word and asynchronisation code word from one pair of code words; or ii) an ID codeword from one pair of code words and a synchronisation code word fromanother pair of code words.

The apparatus may include a microphone for receiving an acoustic signaland the receiver may be configured to receive an audio signal obtainedfrom the microphone.

In one embodiment, the code words are encoded within the audio signalusing one or more of spread spectrum encoding, echo modulation andcritical band encoding so that the code words are unobtrusive to a userlistening to the audio. In the case of echo modulation the polarityand/or time lag and/or amplitude of echoes may be changed based on thedata to be hidden within the audio.

The invention also provides a computer program product comprisingcomputer implementable instructions for causing a programmable computerdevice to perform all the method steps discussed above or to becomeconfigured as the above described apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will become apparent from thefollowing detailed description of exemplary embodiments which aredescribed with reference to the accompanying drawings, in which:

FIG. 1 schematically shows a signalling system for communicating data toa cellular telephone via the audio portion of a television signal;

FIG. 2a is a block schematic diagram illustrating the processingperformed by an audio encoder forming part of the system shown in FIG.1;

FIG. 2b is a block diagram illustrating the main components of an FECencoder forming part of the encoder shown in FIG. 2 a;

FIG. 2c is a timing sequence illustrating the way in whichidentification (ID) codes and synchronisation codes are added to theaudio signal by the encoder shown in FIG. 2 a;

FIG. 2d illustrates the way in which the code words are embedded withinthe audio signal by the encoder shown in FIG. 2 a;

FIG. 3 is a schematic block diagram illustrating the main components ofa cellular telephone including software applications for recovering datahidden within a received audio signal;

FIG. 4a is a schematic block diagram illustrating the main processingmodules of the software application that process the audio signal torecover the data hidden therein;

FIG. 4b is a block diagram illustrating the main processing blocks of acode word extraction module shown in FIG. 4 a;

FIG. 5 is a timing sequence illustrating the beneficial way in which thesoftware application can synchronise with the audio signal using an IDcode word and a synchronisation code word;

FIG. 6a is a timing sequence illustrating the way in which the softwareapplication can re-synchronise with the code words in the audio in theevent of a skip forward event;

FIG. 6b is a timing sequence illustrating the way in which the softwareapplication can re-synchronise with the code words in the audio in theevent of a skip back event;

FIG. 6c is a timing sequence illustrating the way in which the softwareapplication can re-synchronise with the code words in the audio in theevent of a break in the audio coding;

FIG. 6d is a timing sequence illustrating the optional insertion of atleast one additional synchronisation code word in the audio to assist inthe re-synchronisation of the software application with the code wordsin the audio in the event of a break in the audio encoding; and

FIG. 7 illustrates an alternative way that code word pairs may beinserted within the audio signal.

DETAILED DESCRIPTION

Overview

FIG. 1 illustrates a first embodiment of the invention in which codewords F(t), generated by a data source 1, are encoded within an audiotrack from an audio source 3 by an encoder 5 to form a modified audiotrack for a television programme. In this embodiment, the code wordsF(t) include identification code words and synchronising code words forsynchronising the operation of a software application running on auser's mobile telephone 21 with the television programme. When theencoder 5 encodes the code words into the audio, it passes timinginformation to a server 41, which timing information identifies whereeach code word has been embedded within the audio signal (for examplerelative to a start of the audio signal). As will be described in moredetail below, this timing information is used to control the operationof the user's mobile telephone 21 in synchronism with the audio of thetelevision programme. As shown in FIG. 1, the modified audio trackoutput by the encoder 5 is then combined with the corresponding videotrack, from a video source 7, in a signal generator 9 to form atelevision signal conveying the television programme. In thisembodiment, the data source 1, the audio source 3, the video source 7and the encoder 5 are all located in a television studio and thetelevision signal is distributed by a distribution network 11 and, inthis embodiment, a radio frequency (RF) signal 13. The RF signal 13 isreceived by a television aerial 15 which provides the television signalto a conventional television 17. The television 17 has a display (notshown) for showing the video track and a loudspeaker not shown foroutputting the modified audio track as an acoustic signal 19.

As shown, in this embodiment, the cellular telephone 21 detects theacoustic signal 19 emitted by the television 17 using a microphone 23which converts the detected acoustic signal into a correspondingelectrical signal. The cellular telephone 21 then processes theelectrical signal to recover the hidden code words F(t). The cellulartelephone 21 also has conventional components such as a loudspeaker 25,an antenna 27 for communicating with a cellular base station 35, adisplay 29, a keypad 31 for entering numbers and letters and menu keys33 for accessing menu options. The data recovered from the audio signalcan be used, for example, to synchronise a software application runningon the cellular telephone 21 with the television programme being shownon the television 17. For example, there may be a quiz show being shownon the television 17 and the cellular telephone 21 may be arranged togenerate and display questions relating to the quiz that are shown insynchronism with the quiz show. The questions may, for example, beobtained from the server 41 and stored together with timing informationthat defines the timing that the questions are to be output to the userrelative to the timing that the synchronisation codes are recovered fromthe audio. At the end of the quiz show, the answers input by the userinto the cellular telephone 21 (via the keypad 31) can then betransmitted to a remote server 41 via the cellular telephone basestation 35 and the telecommunications network 39. The server 41 can thencollate the answers received from a large number of users and rank thembased on the number of correct answer given and the time taken to inputthe answers. This timing information could also be determined by thecellular telephone 21 and transmitted to the server 41 together with theuser's answers. As those skilled in the art will appreciate, the server41 can also process the information received from the different usersand collate various user profile information which it can store in thedatabase 43. This user profile information may then be used, forexample, for targeted advertising.

After the server 41 has identified the one or more “winning” users,information or a prize may be sent to those users. For example, amessage may be sent to them over the telecommunications network 39together with a coupon or other voucher. As shown by the dashed line 44in FIG. 1, the server 41 may also provide the data source 1 with thedata to be encoded within the audio.

As mentioned above, the inventors have realised that the way in whichthe data is hidden in the audio signal can be improved to make it easierfor the telephone 21 to synchronise with the television programme. Oneimprovement that they have made is to insert pairs of code wordsdistributed throughout the audio programme, with each pair of code wordsincluding an identification (ID) code word and a synchronisation codeword. Typically, the ID code words within a given television programmewill be the same whilst the synchronisation code words are unique withinthe television programme. Different television programmes will havedifferent ID code words and the synchronisation code words used in onetelevision programme may also be used in another television programme.The inventors have found that this arrangement helps to speed upsynchronisation whilst making efficient use of available code words.

Data Hiding

There are various ways in which the code words can be hidden within theaudio signal and the reader is referred to the paper by Bender entitled“Techniques For Data Hiding”, IBM Systems Journal, Vol 35, no 384, 1996,for a detailed discussion of different techniques for hiding data inaudio. In the present embodiment, the data is hidden in the audio byadding an echo to the audio, with the polarity or the time delay of theecho being varied to encode the data. This variation may be performed,for example by using a simple no echo corresponds to a binary zero andan echo corresponds to a binary one scheme. Alternatively, a binary onemay be represented by the addition of an echo at a first delay and abinary zero may be represented by the addition of an echo at a seconddifferent delay. The sign of the echo can also be varied with the datato be hidden. In a more complex encoding scheme a binary one may berepresented by a first combination or sequence of echoes (two or moreechoes at the same time or applied sequentially) and a binary zero maybe represented by a second different combination or sequence of echoes.

More details of such echo encoding techniques can be found in theapplicant's earlier application WO2009/144470, the contents of which arehereby incorporated by reference. However, for completeness, a briefdescription of the encoding process will now be given with reference toFIG. 2. FIG. 2a is a block diagram illustrating the main components ofthe encoder block 5 shown in FIG. 1. As shown, the encoder 5 includes anFEC encoder 71 that encodes each code word separately from the othercode words to generate respective message data for each code word. Themessage data output from the FEC encoder 71 is passed to an echogeneration and shaping module 73, which also receives the audio signalin which the message data is to be hidden. The echo generation andshaping module 73 then generates echoes of the audio which depend uponthe message data to be transmitted. The generated echoes are thencombined with the original audio signal in a combiner module 75. Thecombiner module 75 also records timing information for each code word(or at least for each synchronisation code word) that defines the timepoint within the audio signal that the code word has been encoded (forexample the time from the start of the audio signal to the point in theaudio signal corresponding to the end of the code word). This timinginformation is supplied to the server 41, where the timing informationcan be combined with actions that are to be performed at given timeswithin the audio. The modified audio signal output from the combinermodule 75 is then passed to a gain control module 77 for appropriategain control before the audio signal with the hidden data is ready forcombining with the corresponding video signal.

FIG. 2b is a block diagram illustrating the main components of the FECencoder module 71 used in this embodiment. Each code word that is inputto the FEC encoder 71 is processed separately, so that the encoding andsubsequent decoding of each code word can be independent of the encodingand decoding of other code words. As shown, in this embodiment, the FECencoder 71 has a Reed-Solomon encoder module 81 which uses a shortened(13, 6) block code to represent the input code word. The data outputfrom the Reed-Solomon encoder 51 is then passed to a convolutionalencoder 83 which performs convolutional encoding on the data. The databits output from the convolutional encoder 83 are then interleaved witheach other by a data interleaving module 85 to protect against errorsoccurring in bursts. Finally, a synchronisation data adder module 87adds a sequence of synchronisation bits that will help the telephone 21to lock on to the encoded data within the received acoustic signal 3.The output from the synchronisation data adder module 87 represents themessage data for the input code word which is then passed to the echogeneration and shaping module 73.

FIGS. 2c and 2d are timing sequences illustrating the way in which thecode words are encoded and added into the audio signal 90. Inparticular, in this embodiment, pairs 91-1 to 91-4 of code words areadded. Each pair 91 of code words includes an ID code word 93 and asynchronisation code word 95. As discussed above, the ID code words 93are all the same and the synchronisation code words 95 are all uniquewithin the television programme being encoded. The synchronisation codewords 95 are preferably related to each other—as this can help with thedecoding (as will be discussed below). For example, the synchronisationcode words 95 may increment by a defined amount (e.g. by one) from onesynchronisation code word to the next. In this embodiment, the pairs 91of code words are continuously added to the audio of the televisionprogramme—so that a second pair 91 of code words is added immediatelyafter a first pair 91, until the end of the audio signal. Thisfacilitates the decoding process as once the telephone 21 has found onecode word, it can use the end boundary of that code word as the startboundary for the next code word etc.

As will be described in more detail below, the ID code words 93 allowthe telephone 21 to identify the television programme and hence thecorresponding set of actions that it should take during the programme;and the synchronisation code words 95 allow the telephone 21 tosynchronise the performance of the actions it has to take with thetelevision programme.

Cellular Telephone

FIG. 3 illustrates the main components of the cellular telephone 21 usedin this embodiment. As shown, the cellular telephone 21 includes amicrophone 23 for receiving acoustic signals (such as the user's voiceor the audio of the television programme carrying the hidden code words)and for converting them into electrical equivalent signals. Theseelectrical signals are then filtered by the filter 51 to remove unwantedfrequencies typically outside the frequency band of 300 Hz to 3.4 kHz.The filtered audio is then digitised by an analog to digital converter53, which samples the filtered audio typically at a sampling frequencyof 8 kHz and representing each sample by a 13 to 16 bit digital value.The stream of digitised audio (D(t)) is then input to a processor 63(which may comprise one or more processing units). When making a voicecall, the processor 63 compresses the received audio and then passes itto an RF processing unit 57 which modulates the compressed audio dataonto one or more RF carrier signals for transmission to the base station35 via the antenna 27. Similarly, compressed audio signals received viathe antenna 27 are fed to the RF processing unit 57, which demodulatesthe received RF signals to recover the compressed audio data from the RFcarrier signal(s), which is then passed to the processor 63 fordecompression. The regenerated audio samples are then output to theloudspeaker 25 via the digital to analog converter 59 and the amplifier61.

As shown in FIG. 3a , the processor 63 is controlled by software storedin memory 65. The software includes operating system software 67 (forcontrolling the general operation of the cellular telephone 21), abrowser 68 for accessing the internet and application software 69 forproviding additional functionality to the cellular telephone 21. In thisembodiment, the application software 69 is configured to cause thecellular telephone 21 to interact with the television programme in themanner discussed above. To do this, the application software 69 isarranged to receive and process the audio signal of the televisionprogramme it receives via the microphone 23 to recover the hidden codewords F(t) which controls the application software 69.

FIG. 4a is a block diagram illustrating the main functional modules ofthe software application 69. As shown, the application software 69includes a code word extraction module 101 that processes the audiosignal samples from the A/D converter 53 to recover the code wordshidden therein. FIG. 4b is a block diagram illustrating the maincomponents of the code word extraction module 101. As shown, there is anecho detector 103 that detects echoes in the received audio signal and adata recovery module 105 that processes the echoes to determine datavalues represented thereby. Initially the echo detector 103 does notknow where each symbol period begins and ends or where the start of thedata message is located. Therefore, the echo detector 103 analyses ablock of samples each time that a new sample arrives to determine if itcontains the echoes that carry a hidden data symbol. The output from theecho detector 103 is then analysed by the data recovery module 105 todetermine the most likely symbol boundaries. The data recovery module105 then determines the location of the start of the message by findingthe synchronisation bits that were added by the synchronisation dataadder 87. At this point, the data recovery module 105 can start torecover the whole data message. Once synchronisation has been obtained,because the code words are continuously hidden within the audio, theecho detector 103 can use the determined symbol boundaries and messageboundaries for the current code word to determine approximate symbol andmessage boundaries for the next hidden code word—which reduces theamount of processing required to recover the hidden code words.

The message data output by the data recovery module 105 is thenprocessed by an FEC decoding module 107 which reverses the FEC encodingperformed by the FEC encoder 71 to regenerate the code word which itoutputs. In this embodiment, the FEC decoding module 107 has a blinddecoding mode of operation and a non-blind decoding mode of operation. Acontroller 109 is provided to control the mode of operation of the FECdecoding module 107 in dependence upon decoding control signals. Inparticular, in the blind decoding mode of operation, the FEC decodingmodule 107 processes the incoming data messages without using any apriori information about the possible code word contained therein. Inthe non-blind decoding mode of operation, the FEC decoding module 107uses a priori information about the expected code word to try to improvethe decoding process. In particular, with the alternating structure ofID code word 93 and synchronisation code word 95 used in the presentembodiment, if the previous code word was a synchronisation code word95, then the next code word should be an ID code word 93. Further sincethe same ID code word is used throughout the television programme, thevalue of the expected ID code word should be the same as the value ofthe previous ID code word. Similarly, if the previous code word that wasdecoded was an ID code word, then the next code word should be asynchronisation code word 95; and since the value of one synchronisationcode word is related to the previous synchronisation code word(incremented by one, for example), the expected value of the nextsynchronisation code word can also be determined in advance.Additionally or alternatively, the telephone 21 may know in advance allof the code words that are encoded within the television programme andthe order in which they appear. Therefore, once the telephone 21 hasdecoded one code word, it can determine an expected next code word fromthe pre-stored information. In either case, the FEC decoding module 107can use this knowledge of the expected next code word to control itsdecoding process—for example, using a correlation process, a maximumlikelihood process or a Viterbi process.

In one embodiment, the FEC decoding module 107 normally operates in theblind decoding mode of operation; and if the confidence in the code wordoutput falls below a threshold or if a code word is not detected at all,then the controller 109 switches the FEC decoding module 107 to operatein the non-blind decoding mode of operation. Alternatively, thecontroller 109 may control the operation of the FEC decoding module 107so that it operates in the blind decoding mode until the telephone 21has synchronised itself with the codes in the audio signal, at whichpoint the controller 109 switches the FEC decoding module 107 to operatein its non-blind decoding mode of operation; switching back to the blinddecoding mode if the telephone 21 loses synchronisation with the codewords in the received audio signal.

Returning to FIG. 4a , the extracted code words are passed to an ID codeword processing module 111 and to a synchronisation controller 113. TheID code word processing module 111 ignores the synchronisation codewords and processes the ID code words. In particular, the ID code wordprocessing module 111 initially uses the extracted ID code word 93 toretrieve an actions and timings file 117 from the database 115. Theactions and timings file 117 obtained from the database 115 are specificto the television programme in which the code words have been hidden.The database 115 may store actions and timings files 117 for a range ofdifferent television programmes that were or are scheduled to bebroadcast over a given period—such as over the last week and the next 24hours. The timings are the timings identified by the encoder 5 duringthe encoding of the audio signal (or are timings derived therefrom) andthe actions are the actions determined by the server 41 to be performedby the telephone 21 during the television programme. The actions andtimings file 117 for each television programme may be downloaded fromthe server 41 to the database 115 over the wireless radio link or theymay be provided to the telephone 21 on a memory card.

As discussed above, the ID code words 93 in a television programme arethe same. Therefore, if the ID code word processing module 103 detects anew ID code word, then it assumes that the user has changed channel andit informs the synchronisation controller 105 accordingly. The ID codeword processing module 111 also stops the operation of the outputcontroller 121. The ID code word processing module 111 may wait for thenext ID code word 93 to confirm the change of ID code word beforestopping the output controller 121—just in case the detected new ID codeword was erroneously decoded. If the new ID code word is confirmed, thenthe ID code word processing module 103 retrieves the actions and timingsfile 117 for the new television programme corresponding to the new codeword that has been detected from the local database 115 (if it isavailable) or from a remote database (not shown) if it is not storedlocally.

The synchronisation controller 113 also receives the code words outputfrom the code word extraction module 101 and the actions and timingsfile 117 retrieved from the database 115. The synchronisation controller113 uses the synchronisation code words 95 that are detected in theaudio signal to control a timer 119 used to control the time when theactions (defined in the actions and timings file 117) should beperformed. In particular, the actions and timings file 117 for thetelevision programme include a list of actions to be performed by thetelephone 21 and the time when each action should be performed. The timeis defined, for example, relative to the start of the televisionprogramme or relative to a previous event. The timer 119 is synchronisedto that start point or to the previous event, so that the outputcontroller 121 can output the action to be performed at the correcttiming. For example, the actions and timings file 117 may define that agiven action should take place 12 minutes and 23 seconds from the startof the television programme. The timer 119 is synchronised with thestart of the show so that the relevant action is performed at thecorrect time. Whilst it is possible to set the timer 119 at the start ofthe television programme and let it “free wheel” to control theoutputting of the actions, since the user may be able to control theplay back speed (for example using pause, fast forward or rewind etc.),the actions and timings file 117 includes the timing of when eachsynchronisation code word 95 appears in the audio signal. In this way,the synchronisation controller 113 can compare the actual timer valuewhen a synchronisation code word 95 is received with the expected timervalue (as defined in the actions and timings file 117) and change thetimer 119 using the difference in time between the actual time and theexpected time. In this way, if the user does skip sections of thetelevision programme, the timer 119 will be updated upon receipt of thenext synchronisation code word, so that the relevant actions are outputand performed at the correct timing.

Since the telephone 21 may be located in a noisy environment, some ofthe code words may not be detected by the code word extraction module101. Therefore, the output controller 121 is designed to output thesubsequent actions in accordance with the actions and timing file 117when the timer 119 shows the corresponding time. In this way, theperformance of a given action does not depend on correctly detecting acorresponding synchronisation code word. Action triggering has beenseparated from the task of correctly decoding the code words.

In this embodiment, since the synchronisation controller 113 has theactions and timings file 117, it knows what synchronisation code wordsto expect. If a synchronisation code word is received that does notmatch one of the expected code words, then the synchronisationcontroller 113 will stop the operation of the output controller 121. Itmay do this immediately or it may wait until after decoding the nextsynchronisation code word to see if the last code word was an error orif the user has perhaps changed channel and is now watching anothertelevision programme—that is also encoded with similar code words.

When the telephone 21 has synchronised with the code words in theincoming audio, the synchronisation controller 113 also determines andsends the next expected code word to the code extraction module 101 (asthe decoding control signals) for use in controlling the way in whichthe FEC decoding module 107 decodes the message data (using the blind orthe non-blind decoding mode of operation discussed above).

The actions output by the output controller 121 cause the applicationsoftware 69 to perform the defined action—such as to generate and outputdata (such as questions for the user) on the display 29 and to receivethe answers input by the user via the keypad 31. The softwareapplication 69 then transmits the user's answers to the remote server 41(identified by a pre-stored URL, E.164 number or the like) together withtiming data indicative of the time taken by the user to input eachanswer (calculated by the software application 69 using an internaltimer (not shown)). The software application 69 may also display resultinformation received back from the server 41 indicative of how well theuser did relative to other users who took part in the quiz.

Advantages

As discussed above, one of the advantages of inserting pairs of ID codewords 93 and synchronisation code words 95 is that it allows for fastsynchronisation to the audio stream. In particular, because the ID codewords 93 are encoded into the audio separately from the synchronisationcode words 95, the synchronisation controller 113 can synchronise to theincoming audio by detecting an ID code word 93 and then asynchronisation code word 95 or by detecting a synchronisation code word95 and then an ID code word 93. This can result in a significant savingin terms of the time needed to achieve synchronisation. In particular,with the echo encoding process used in this embodiment (which has a verylow bit rate), each code word may be encoded over approximately 5 to 30seconds of the television audio, depending on the encoding performed bythe FEC encoder. If every code word were to be unique and the samecoding space is assumed (i.e. a code space having the same number ofunique code words as there are unique combinations of code word pairs91) then every code word would need to be about twice the length of thecode words used in this embodiment. Therefore, if the telephone 21started to try to decode the audio just after the start of one such longcode word, the telephone 21 would have to wait until the end of the nextfull code word before it will be synchronised with the audio. However,by splitting up the long code words into two shorter code words—a commonID code word and a unique synchronisation code word, and by allowing thesynchronisation controller 113 to synchronise onto an ID code word 93and then a synchronisation code word 95 or a synchronisation code word95 and then an ID code word 93, the worst case delay in achievingsynchronisation can be reduced by a quarter.

In addition to the benefit of fast synchronisation when the telephone 21starts the decoding process, the split approach to the code words usedin this embodiment also facilitate fast re-synchronisation—for exampleif the user skips ahead in the audio stream or skips back or if there isa break in the encoding. Under these circumstances, the nextsynchronisation code word received by the code word extraction module101 and forwarded to the synchronisation controller 113 will appear outof order and therefore cause the timer 119 to be reset to theappropriate point in the audio as described earlier. These scenarios areillustrated in FIG. 6.

In particular, FIG. 6a illustrates a scenario where a user decides toskip ahead at time t1 to time t2 (corresponding to the middle of an IDcode word 93). In this embodiment, however, the synchronisationcontroller 113 assumes that the user has not changed televisionprogramme and so the ID code words 93 will be the same. Therefore, thesynchronisation controller 113 can re-synchronise with the audio afterdecoding the next synchronisation code word 95 (in this case SYNC12) attime t3.

FIG. 6b illustrates a scenario where a user decides to skip back at timet4 to time t5 (corresponding to the middle of an ID code word 93).Again, the synchronisation controller 113 assumes that the user has notchanged television programme and so the ID code words 93 will be thesame. Therefore, the synchronisation controller 113 can re-synchronisewith the audio after decoding the next synchronisation code word 95 (inthis case SYNC11) at time t6. Also by comparing the synchronisation codeword after re-synchronisation with the previous synchronisation codeword, the system can detect that the user has skipped forwards/backwardsin the media programme.

FIG. 6c illustrates a scenario where a break in the coding is insertedin the audio signal. Such breaks may occur because this part correspondsto a portion of the television audio that is very quiet or it maycorrespond to a point at which adverts have been inserted into thetelevision programme. In this case the break in the encoding starts attime t7 and ends at time t8. The synchronisation controller 113synchronises with the audio again at time t9 after decoding thesynchronisation code (SYNC21). As can be seen from FIG. 6c , the firstcode word after the end of a break is preferably a synchronisation codeword 95—as this minimises the time that the synchronisation controller113 will take to re-synchronise with the audio of the televisionprogramme after the break. In cases where there are known break pointswithin the programme for adverts, the database 115 may store anindication that synchronisation loss occurs at the break point. If theduration of the advert break is known, then the database may also storedata indicating when the next code word is to be found. However, in manycases although the break point will be known in advance, the length ofthe advert break will not be known in advance.

Additionally, in the preferred embodiment, one or more additionalsynchronisation code words 95 are inserted after a break—as this allowsthe synchronisation controller 113 to re-synchronise quickly even if thefirst one or two code words can't be detected in the audiosignal—perhaps because the television programme has been altered toadjust for a desired length of advert break whilst keeping thetelevision programme to a set duration in the television schedule; orbecause background noise (which tends to be higher during advert breaks)prevented the correct decoding of the first code word after the break.Such additional synchronisation code words are illustrated in FIG. 6d ,which shows two consecutive synchronisation code words (SYNC21 andSYNC22) after the break. In this example, the first code word may not bedetected, in which case the synchronisation controller 113 canre-synchronise with the audio after decoding SYNC22 at time t10.

Modifications and Further Alternatives

A number of embodiments have been described above illustrating the wayin which ID code words 93 and synchronisation code words 95 are hiddenwithin an audio signal in code word pairs. As those skilled in the artwill appreciate various modifications and improvements can be made tothe above embodiments and some of these modifications will now bedescribed.

In the above embodiments, the pairs of code words were continuouslyembedded within the audio signal. This is not essential. The pairs ofcode words may be distributed within the audio signal such that thereare gaps (time periods in which the audio signal does not carry hiddendata) between adjacent pairs of code words. Such an arrangement isillustrated in FIG. 7, which also shows that the order of the code wordsin each pair does not need to be the same. This may be useful wheninserting a code word pair after a break—so that the first code wordafter the break is a synchronisation code word. In the above embodiment,the pairs of code words were added to the audio in an alternatingfashion. Whilst this is preferred, it is not essential, as additionalcode words may be added between the pairs of code words. An example ofsuch an embodiment is illustrated in FIG. 6 d.

In the above embodiments, the various processing of the audio sampleswas performed by software running on the cellular telephone. As thoseskilled in the art will appreciate, some or all of this processing maybe formed by dedicated hardware circuits, although software is preferreddue to its ability to be added to the cellular telephone aftermanufacture and its ability to be updated once loaded. The software forcausing the cellular telephone to operate in the above manner may beprovided as a signal or on a carrier such as compact disc or othercarrier medium. Additionally, instead of using a cellular telephone toperform the various processing discussed above, other portable devicesmay be used, such as laptop computers, PDAs, tablet computers and thelike.

In the above embodiment, the television programme was transmitted to theuser via an RF communication link 13. As those skilled in the art willappreciate, the television programme may be distributed to the user viaany appropriate distribution technology, such as by cable TV, theInternet, Satellite TV etc. It may also be obtained from a storagemedium such as a DVD or BD and read out by an appropriate DVD/BD player.

In the above embodiments, the cellular telephone picked up the audio ofa television programme. As those skilled in the art will appreciate, theabove techniques can also be used where the audio is obtained from aradio or other loudspeaker system, such as in cinemas or in other livestage shows.

In the above embodiments, it was assumed that the code words were hiddenwithin the audio at the television studio end of the television system.In an alternative embodiment, the code words may be hidden within theaudio at the user's end of the television system, for example, by a settop box during play-out or during recording if the programme is beingrecorded locally. The set top box may be adapted to hide the appropriatedata into the audio prior to outputting the television programme to theuser.

In the above embodiment, synchronisation bits were added to the datathat was transmitted so that the decoder can identify the boundaries ofeach symbol period and the start and end of each message. The use ofsuch synchronisation bits significantly increases the overall messagelength that has to be transmitted (in some cases by as much as 25%).Additionally, as the decoding of each bit is subject to noise, thematching is not perfect which can reduce the chances of a successfulsynchronisation. As described in the applicant's earlier patentapplication WO2009/144470, the synchronisation bits are not required. Inparticular, the FEC decoding module 41 will have higher error rates whenthe echo detector 103 is not properly synchronised with the incomingdata compared with its error rate when the echo detector is synchronisedwith the incoming data. Therefore, it is possible to use the error rateoutput from the FEC decoding module 107 to control the synchronisationof the receiver to the incoming data.

As those skilled in the art will appreciate, the techniques describedabove for hiding data within the audio may be done in advance of thetransmission of the acoustic signal or it may be done in real time. Evenin the case where the data is to be embedded within an audio signal inreal time, some of the processing can be done in advance. For example,the FEC encoding may be performed on the data in advance so that onlythe echo generation and echo shaping is performed in real time.

In order to keep the obtrusiveness of the added echoes to a minimum, thedata rate of the encoded data is preferably kept between one and twentysymbols per second. This corresponds to a symbol period of between 50 msand 1 second. In some embodiments, a long symbol period is beneficialbecause the added echoes will span across spoken words within the audio,making it easier to hide the data echoes within the audio. A largersymbol period also reduces audibility of the echoes. This is becausehumans are more sensitive to changing echoes than they are to static orfixed echoes. Therefore, by having a longer symbol period, the rate ofchange of the echoes is lower making the presence of the echoes lessnoticeable to a user.

Typically, the data rate of the data added to the audio signal in thetransmitter is constant and is known by the receiver. This knowledgereduces the complexity of the receiver circuitry for locking on to thedata within the received signal. However, it is not essential to theinvention and more complex circuitry may be provided in the receiver toallow the receiver to try different data rates until the actual datarate is determined. Similarly, the receiver may use other techniques tosynchronise itself with the transmitted data so that it knows where thesymbol boundaries are in advance of receiving the data.

In the above embodiment, FEC encoding techniques were used to allow thereceiver to be able to correct errors in the received data. As thoseskilled in the art will appreciate, such encoding techniques are notessential to the invention. However, they are preferred, as they help tocorrect errors that occur in the transmission process over the acousticlink.

In the embodiment described above, a single transmitter was providedtogether with a receiver. As those skilled in the art will appreciate,multiple transmitters and/or multiple receivers may be provided.Further, the components of the transmitter may be distributed among anumber of different entities. For example, the encoding and data hidingpart of the transmitter may be provided within a head end of atelevision distribution system or a user's set top box.

In a further modification, multiple data code words may be encoded “ontop of each other”—at the same point in the audio, for example usingdifferent echo lags, provided the code words do not interfere with eachother. The way in which this can be achieved is described in theapplicant's earlier applications discussed above.

In the above embodiments, a number of processing modules and circuitshave been described. As those skilled in the art will appreciate, theseprocessing modules and circuits may be provided as hardware circuits oras software modules running within memory of a general purposeprocessor. In this case, the software may be provided on a storagemedium such as a CD-ROM or it may be downloaded into an appropriateprogrammable device on a carrier signal over a computer network, such asthe internet. The software may be provided in compiled form, partiallycompiled form or in un-compiled form.

As those skilled in the art will appreciate, the precise values of thebit rates, sampling rates etc described in the above embodiments are notessential features of the invention and can be varied without departingfrom the invention.

The invention claimed is:
 1. A method of synchronising a device with anaudio signal having encoded pairs of code words embedded separatelytherein, each pair of code words including an ID code word and asynchronisation code word, the method comprising: receiving the audiosignal with the encoded pairs of code words; synchronising the devicewith the audio signal by detecting: i) an ID code word and asynchronisation code word from one pair of code words; or ii) an ID codeword from one pair of code words and a synchronisation code word fromanother pair of code words; and using a detected ID code word toretrieve timing information relating to an expected time for detectedsynchronisation code words and determining a difference in time betweenan expected time for a detected synchronisation code word and an actualtime of the detected synchronisation code word and using the determineddifference to synchronise the device to the audio signal.
 2. A methodaccording to claim 1, wherein the timing information defines an expectedtime relative to a defined point within the audio signal, such as thestart of the audio signal.
 3. A method according to claim 1, comprisingusing the determined difference to alter a timer of the device.
 4. Amethod according to claim 1, further comprising storing actioninformation that defines one or more actions that the device should takeat defined times during the reception of the audio signal.
 5. A methodaccording to claim 1, comprising using a first decoding technique todecode the encoded code words in the received audio signal beforesynchronisation and using a second decoding technique to decode theencoded code words after synchronisation.
 6. A method according to claim1, comprising using a first decoding technique to decode the encodedcode words in the received audio signal and using a second decodingtechnique to decode the encoded code words in the event that an expectedcode word is not detected.
 7. A non-transitory computer program productcomprising computer implementable instructions for causing aprogrammable computer device to perform all the method steps of claim 1.8. A method of synchronising a device with an audio signal havingencoded pairs of code words embedded separately therein, each pair ofcode words including an ID code word and a synchronisation code word,the method comprising: receiving the audio signal with the encoded pairsof code words; synchronising the device with the audio signal bydetecting: i) an ID code word and a synchronisation code word from onepair of code words; or ii) an ID code word from one pair of code wordsand a synchronisation code word from another pair of code words; using afirst decoding technique to decode the encoded code words in thereceived audio signal before synchronisation and using a second decodingtechnique to decode the encoded code words after synchronization;wherein the first decoding technique is a blind decoding technique andthe second decoding technique is a non-blind decoding technique.
 9. Amethod of synchronising a device with an audio signal having encodedpairs of code words embedded separately therein, each pair of code wordsincluding an ID code word and a synchronisation code word, the methodcomprising: receiving the audio signal with the encoded pairs of codewords; synchronising the device with the audio signal by detecting: i)an ID code word and a synchronisation code word from one pair of codewords; or ii) an ID code word from one pair of code words and asynchronisation code word from another pair of code words; wherein aftersynchronising the device to the audio and in response to a skip withinthe received audio signal, maintaining the ID code word andre-synchronising with the audio signal by detecting a synchronisationcode word.
 10. An apparatus for synchronising a device with an audiosignal having encoded pairs of code words embedded separately therein,each pair of code words including an ID code word and a synchronisationcode word, the method comprising: a receiver configured to receive theaudio signal with the encoded pairs of code words; and a synchronisationcontroller configured to synchronise the device with the received audiosignal by detecting: i) an ID code word and a synchronisation code wordfrom one pair of code words; or ii) an ID code word from one pair ofcode words and a synchronisation code word from another pair of codewords; a code word extracting module that uses a first decodingtechnique to decode the encoded code words in the received audio signalbefore synchronisation and that uses a second decoding technique todecode the encoded code words after synchronization; wherein the firstdecoding technique is a blind decoding technique and the second decodingtechnique is a non-blind decoding technique.
 11. An apparatus forsynchronising a device with an audio signal having encoded pairs of codewords embedded separately therein, each pair of code words including anID code word and a synchronisation code word, the method comprising: areceiver configured to receive the audio signal with the encoded pairsof code words; and a synchronisation controller configured tosynchronise the device with the received audio signal by detecting: i)an ID code word and a synchronisation code word from one pair of codewords; or ii) an ID code word from one pair of code words and asynchronisation code word from another pair of code words; an ID codeword processing module that uses a detected ID code word to retrievetiming information relating to an expected time for detectedsynchronisation code words; and wherein the synchronisation controlleris arranged to determine means for determining a difference in timebetween an expected time for a detected synchronisation code word and anactual time of the detected synchronisation code word and to use thedetermined difference to synchronise the user device to the audiosignal.
 12. An apparatus according to claim 11, wherein the timinginformation defines an expected time relative to a defined point withinthe audio signal, including the start of the audio signal.
 13. Anapparatus according to claim 11, wherein the synchronisation controlleris arranged to use the determined difference to alter a timer of thedevice.
 14. An apparatus according to claim 11, further comprisingstorage that stores action information that defines one or more actionsthat the device should take at defined times during the reception of theaudio signal.
 15. An apparatus according to claim 11, comprising a codeword extracting module that uses a first decoding technique to decodethe encoded code words in the received audio signal beforesynchronisation and that uses a second decoding technique to decode theencoded code words after synchronisation.
 16. An apparatus according toclaim 11, comprising a code word extracting module that uses a firstdecoding technique to decode the encoded code words in the receivedaudio signal and that uses a second decoding technique to decode theencoded code words in the event that an expected code word is notdetected.
 17. An apparatus according to claim 11, wherein thesynchronisation controller is arranged to re-synchronise the device withthe audio signal in response to a skip event within the received audiosignal by maintaining a previously decoded ID code word and by detectinga new synchronisation code word after the skip event.
 18. An apparatusaccording to claim 11, comprising a microphone for receiving an acousticsignal and wherein the receiver is configured to receive an audio signalobtained from the microphone.